Conventionally, an air-fuel ratio control apparatus has been widely known, which comprises a three-way catalytic converter disposed in an exhaust passage of an internal combustion engine, and an upstream air-fuel ratio sensor and a downstream air-fuel ratio sensor disposed, in the exhaust passage, upstream and downstream of the three-way catalytic converter, respectively. The air-fuel ratio control apparatus performs a feedback control on an air-fuel ratio (air-fuel ratio of the engine) of a mixture supplied to the engine based on the output value of the upstream air-fuel ratio sensor and the output value of the downstream air-fuel ratio sensor in such a manner that the air-fuel ratio of the engine coincides with (becomes equal to) a stoichiometric air-fuel ratio.
Such a air-fuel ratio control apparatus carries out the feedback control on the air-fuel ratio of the engine using a control amount (air-fuel ratio feedback amount) commonly used for all of the cylinders. That is, the air-fuel ratio feedback control is performed in such a manner that an average of the air-fuel ratio of the mixture supplied to the entire engine becomes equal to the stoichiometric air-fuel ratio.
For example, when a measured value or an estimated value of an intake air amount of the engine deviates from a “true intake air amount”, a fuel injection amount calculated based on the measured value or the estimated value of the intake air amount deviates from an “amount required to have the air-fuel ratio coincide with the stoichiometric air-fuel ratio.” Consequently, each of the air-fuel ratios of the cylinders uniformly (evenly) is deviated toward “a rich side or a lean side” with respect to the stoichiometric air-fuel ratio. In this case, the conventional air-fuel control shifts the air-fuel ratio of the mixture supplied to the engine to “a lean side or a rich side”. As a result, each air-fuel ratio of the mixture supplied to each cylinder is corrected to become in the vicinity of the stoichiometric air-fuel ratio. Accordingly, a combustion state in each of the cylinders comes closer to a perfect combustion state (the combustion state when the air-fuel ratio of the mixture is equal to the stoichiometric air-fuel ratio), and an air-fuel ratio of an exhaust gas flowed into the three-way catalytic converter becomes equal to the stoichiometric air-fuel ratio or in the vicinity of the stoichiometric air-fuel ratio. This can avoid worsening of an emission.
Meanwhile, an electronic control fuel injection type internal combustion engine typically comprises at least one fuel injector in each of the cylinders or in each of intake ports, each communicating with each of the cylinders. Accordingly, when a property (characteristic) of the fuel injector for a specific cylinder becomes a “property that the fuel injector injects fuel in an amount larger (more excessive) than an instructed fuel injection amount”, only an air-fuel ratio (air-fuel-ratio-of-the-specific-cylinder) of a mixture supplied to the specific cylinder shifts (deviates) to an extremely richer side. That is, a non-uniformity among air-fuel ratios of the cylinders (variation in air-fuel ratios among the cylinders, air-fuel ratio imbalance among the cylinders) becomes high (prominent). In other words, there arises an imbalance among the air-fuel ratios (hereinafter, each referred to as an “individual cylinder air-fuel ratio”) of the mixtures supplied to a plurality of the cylinders.
In this case, the average of the air-fuel ratios of the mixtures supplied to the engine becomes an air-fuel ratio richer than the stoichiometric air-fuel ratio. Accordingly, the feedback amount commonly used for all of the cylinders causes the air-fuel ratio of the specific cylinder to shift to a leaner air-fuel ratio so that the air-fuel ratio of the specific cylinder is made closer to the stoichiometric air-fuel ratio. Further, each of the air-fuel ratios of the other cylinders is caused to shift to a leaner air-fuel ratio so that the air-fuel ratios of the other cylinders are made to deviate more from the stoichiometric air-fuel ratio. In this case, the air-fuel ratio of the specific cylinder is still richer than the stoichiometric air-fuel ratio, and the air-fuel ratios of the other cylinders become leaner than the stoichiometric air-fuel ratio. Therefore, the average of the air-fuel ratios of the entire mixtures supplied to the engine is made to coincide with the stoichiometric air-fuel ratio.
However, since the air-fuel ratio of the specific cylinder is still richer than the stoichiometric air-fuel ratio, and the air-fuel ratios of the other cylinders are leaner than the stoichiometric air-fuel ratio, the combustion condition of the mixture in each of the cylinders is different from the perfect combustion condition. As a result, an amount of emissions (an amount of unburnt substances and an amount of nitrogen oxides) discharged from each of the cylinders increases. Accordingly, even though the average of the air-fuel ratios of the mixtures supplied to the engine is equal to the stoichiometric air-fuel ratio, the three-way catalytic converter may not be able to purify the increased emissions, and thus, there is a possibility that the emissions become worse. It is therefore important to detect whether or not the air-fuel ratio non-uniformity among cylinders becomes excessively large so that some appropriate measure can be taken in order not to worsen the emissions.
One of such conventional apparatuses for determining whether or not a non-uniformity of the air-fuel ratios of the air-fuel mixtures among the cylinders (an air-fuel ratio imbalance among cylinders, non-uniformity among the individual cylinder air-fuel ratios) becomes excessively large (air-fuel ratio imbalance among cylinders determining apparatus) obtains an estimated air-fuel ratios, each representing each of the air-fuel ratios of the cylinders, by analyzing the output of a single air-fuel ratio sensor (upstream air-fuel ratio sensor) disposed at an exhaust gas aggregated portion. Then the conventional apparatus determines whether or not the “non-uniformity of the air-fuel ratios among the cylinders” becomes excessively large using the estimated air-fuel ratios (refer to, for example, Japanese Patent Application Laid-Open No. 2000-220489). It should be noted that the determination of whether or not the air-fuel ratio imbalance state among cylinders is occurring may be referred to as an “air-fuel ratio imbalance among cylinders determination” or an “imbalance determination”, in the present specification.
Further, another of the conventional apparatuses obtains a trajectory length of or a frequency of a variation in the output signal (output value) of the air-fuel ratio sensor (upstream air-fuel ratio sensor) disposed at the exhaust gas aggregated portion into which the exhaust gas discharged from a plurality of the cylinders aggregate/merge, compares the trajectory length or the frequency of a variation with a “reference value varying depending on the engine rotational and the intake air amount”, and determines whether or not the air-fuel ratio imbalance state among cylinders is occurring based on the comparison result (refer to, for example, U.S. Pat. No. 7,152,594).
On one hand, various proposals have been made concerning a variable compression ratio internal combustion engine with a variable compression ratio mechanism which varies a “mechanical compression ratio”, which is a ratio of a “volume of the combustion chamber when the piston is at a bottom dead center” to a “volume of the combustion chamber when the piston is at a top dead center”. These types of the variable compression ratio internal combustion engines may vary the mechanical compression ratio using any one of techniques described below.
(1) Varying a movement distance of a piston (movement distance of the piston between when the piston is at the bottom dead center and when the piston is at the top dead center) using a link mechanism (refer to, for example, Japanese Patent Application Laid-Open No. 2004-239147).
(2) Varying an inclination angle of a cylinder block to a crank case.
(3) Moving the cylinder block with respect to the crank case along an axial direction of the cylinder (refer to, for example, Japanese Patent Application Laid-Open No. 2003-206771, and Japanese Patent Application Laid-Open No. 2007-303423).
(4) Varying a distance between a piston and a crank shaft (refer to, for example, Japanese Patent Application Laid-Open No. Hei 2-163429).
On the other hand, various proposals have also been made concerning a variable compression ratio internal combustion engine which can vary a “substantial compression ratio”, which is a ratio of a “volume of the combustion chamber at a intake valve closing timing when a substantial compression of the mixture starts” to a “volume of the combustion chamber when the piston is at the top dead center” (refer to, for example, Japanese Patent Application Laid-Open No. 2007-303423). It should be noted that, in the present specification, a “compression ratio” includes both the mechanical compression ratio and the substantial compression ratio.